To produce castings by gravity pouring, a casting mold composed of sand particles, which is a gas-permeable casting mold (a so-called sand mold), is most generally used. With such a gas-permeable casting mold, which may be simply called “mold,” a gas (generally air) remaining in a cavity of a particular shape is pushed out of the cavity by a metal melt (simply called “melt”), and the melt is formed into a casting having substantially the same shape as the cavity. The cavity of the casting mold generally includes a sprue, a runner, a feeder and a product-forming cavity, into which a melt is supplied in this order. When a melt head in the sprue becomes high enough to fill a product-forming cavity, the pouring of the melt is finished.
A solidified melt forms a casting integrally extending from the sprue to the runner, the feeder and the product-forming cavity. The feeder is not an unnecessary portion for obtaining sound castings, while the sprue and the runner are merely paths for a melt to reach the product-forming cavity, which need not be filled with the melt. Thus, as long as a melt is solidified in a state of filling the sprue and the runner, drastic improvement in a pouring yield cannot be expected. In the case of castings integrally having unnecessary portions, considerable numbers of steps are needed to separate cast products from unnecessary portions, resulting in low production efficiency. Accordingly, the sprue and the runner pose large problems in increasing efficiency in gravity casting.
A revolutionary method for solving the above problems is proposed by JP 2007-75862 A and JP 2010-269345 A. To fill a desired cavity portion, part of a cavity in a gas-permeable casting mold, this method pours a metal melt in a volume smaller than that of an entire cavity in a gas-permeable casting mold (hereinafter referred to as “casting mold cavity”) and substantially equal to that of the desired cavity portion, into the cavity by gravity; supplies a gas (compressed gas) into the cavity through a sprue before the melt fills the desired cavity portion; and then solidifies the melt filling the desired cavity portion. By this method commonly disclosed in JP 2007-75862 A and JP 2010-269345 A, which may be called “pressure-casting method,” it is expected to make it substantially unnecessary to fill a sprue and a runner with a melt, because pressure to be obtained by the melt head height is given by the compressed gas.
As a result of experiment to follow the pressure-casting method described in JP 2007-75862 A and JP 2010-269345 A, the inventors have found that in the method of JP 2007-75862 A for closing a sprue with a flange of a gas-supplying pipe to prevent the leak of a compressed gas supplied through the sprue, the positioning of the gas-supplying pipe in the sprue is difficult because the flange conceals the sprue, likely resulting in a slow gas-supplying timing and cold shut in products. In addition, because melt droplets scattered while pouring are likely attached to the sprue in contact with the flange, providing a gap between the flange and the sprue, a large amount of a gas may leak. It is thus desired to develop a means capable of supplying a gas quickly and surely after pouring a melt.